The invention relates to an a.c. voltage regulator employing a thyristor, and more particularly a phase controlled voltage regulator of the type employing a three terminal thyristor.
A conventional a.c. voltage regulator of the type described is constructed as illustrated in FIG. 1 or FIG. 3 Referring to FIG. 1, connected in series across the opposite terminals 1a, 1b of an a.c. power supply 1 are a load 2 and a three terminal, bilateral thyristor 3. The thyristor 3 is associated with a time constant circuit comprising a series combination of a variable resistor 7 and a capacitor 8. The variable resistor 7 has its one end connected with a second anode terminal 4 of the thyristor 3 while the capacitor 8 has its one end connected with a first anode terminal 5 of the thyristor 3, with the junction 9 between the resistor 7 and the capacitor 8 being connected with a gate terminal 6 of the thyristor 3 through a bilateral diode 10 which serves to produce a trigger pulse. The three terminal, bilateral thyristor 3 may for example, be a TRIAC (trademark of General Electric Company) and the bilateral diode 10 may, for example be a DIAC (trademark of General Electric Company) which is a two terminal, bilateral thyristor.
In the operation of the conventional voltage regulator shown, the thyristor 3 initially remains non-conductive, so that a current flows to charge the capacitor 8 through the resistor 7. As the voltage across the capacitor 8 reaches a breakover voltage of the bilateral diode 10, the latter conducts, applying a trigger pulse, which is synchronized with each one-half cycle of the supply frequency, to the gate terminal 6 of the thyristor 3. Thereupon, a conduction path is established across the anode terminals 4, 5, whereby the current from the supply 1 flows through the load 2 and the thyristor 3. By changing the time constant established by the resistor 7 and the capacitor 8 to vary the phase angle at which a trigger pulse is produced during each one-half cycle of the alternating current, the conduction angle of the thyristor 3 can be controlled, thereby providing a control of the effective a.c. voltage across the load 2.
Another conventional circuit as shown in FIG. 3 comprises a full wave rectifier 18 comprising diodes 18a to 18d and having its input terminals 19a, 19b connected across an a.c. supply 21 in series with a load 22. The full wave rectifier 18 has a pair of output terminals 20a, 20b across which is connected a three terminal, unilateral thyristor 23 which is associated with a time constant circuit comprising a series combination of a variable resistor 27 and a capacitor 28. The resistor 27 has its one end connected with an anode terminal 24 of the thyristor 23 while the capacitor 28 has its one end connected with a cathode terminal 25 of the thyristor 23. The junction 29 between the resistor 27 and the capacitor 28 is connected with a gate terminal 26 of the thyristor 23 through a two terminal, unilateral thyristor 30 which serves to produce a trigger pulse.
The operation of the circuit shown in FIG. 3 is similar to that described previously in connection with FIG. 1. Specifically, initially the capacitor 28 is charged through the load 22 and the variable resistor 27, and as the voltage across the capacitor 28 reaches the breakover voltage of the unilateral thyristor 30, the latter conducts to apply a trigger pulse, as synchronized with each one-half cycle of the supply frequency, to the gate terminal 26 of the thyristor 23. Thereupon a conduction path is established across the anode terminal 24 and the cathode terminal 25 of the thyristor 23, permitting a current flow from the supply 21 through the load 22. By changing the time constant established by the resistor 27 and the capacitor 28 to vary the phase angle at which a trigger pulse is produced during each one-half cycle of the alternating current, the conduction angle of the thyristor 23 can be controlled, thereby providing a control over the effective a.c. voltage across the load 22.
However, the conventional circuits shown in FIGS. l and 3 have disadvantages in that, as the supply voltage increases, the changing current to the capacitor 8, 28 increases. This increase in the charging current causes the breakover voltage of the trigger pulse producing element, that is, either diode 10 or unilateral thyristor 30, to be reached earlier in time to increase the conduction angle of the thyristor 3, 23, thereby causing an unintended increase in the effective voltage across the load 2, 22. If the supply voltage decreases, the effective voltage across the load 2, 22 will also decrease. In this manner, a voltage fluctuation of the power supply produces an amplified effect upon the effective voltage across the load. In one experiment, it is found that when a supply voltage of 100V is used and the circuit parameters are chosen to establish an effective voltage across the load 2 to 30V, a fluctuation of .+-.10% from the nominal voltage of 100V resulted in a variation in the effective voltage across the load 2 which is as high as .+-.100% from the designed value of 30V.